// const el = document.getElementById('why') as HTMLImageElement
// el.src = 'url地址'

class person {

}

class Student extends person {
   studying () {
     console.log('studying')
   }
}

function sayHello (p: person) {
  (p as Student).studying()
}
const stu = new Student()
sayHello(stu)

const message: string = 'hello world'
let mes = (message as unknown) as number
mes = 123
console.log(mes)
